1. Field of the Invention
The present invention relates to a mixer stage with an oscillator, a first input, a second input, a first output, a second output, a set of four controllable amplifier elements, and a first current source.
The invention further relates to a method for mixing a first signal which has a first frequency, with an oscillator signal, which has a second frequency, having the steps of: supplying the first signal in differential form to a first input and a second input of a mixer stage with an oscillator, a first output, a second output, a set of four controllable amplifier elements, and a first current source.
2. Description of the Background Art
A mixer stage is known in the form of a Gilbert cell, and a method is know through the operation of the Gilbert cell.
Since the invention of the Gilbert cell by Brad Gilbert in 1969, with some few exceptions, mixer cells have been implemented by a structure consisting of three stages wired in series. The first stage consists of a current source, or in the simpler case, a resistor, the second stage consists of a differential amplifier implemented using bipolar or unipolar technology, and the third stage has four switching transistors that invert the output current of the differential amplifier at the rate of an oscillator frequency. It is also known to use negative feedback resistors at the emitters of the switching transistors to improve the linearity of the input/output signal characteristics.
Modern semiconductor technologies reduce the feature sizes in all dimensions with the goal of increasing the packing density of circuits and the achievable cutoff frequencies. This is associated with a reduction in the dielectric strength. This phenomenon is familiar from different generations of personal computers, which are provided with steadily increasing clock frequencies and steadily reduced operating voltages. These technologies are also used in the communications field.
However, that which is possible in digital circuit technology causes problems in analog technology. The series circuit of three stages or circuit sections required in the Gilbert cell permits only a minimum voltage for each stage or circuit section. If one extrapolates this effect into the future, a point in time will soon be reached when it will no longer be possible to implement a Gilbert cell with further decreases in supply voltage.
In other applications, for example in cell phones, the supply voltage must be limited to the 3V that can be supplied by a single battery. When one subtracts the requirement for voltage regulation and swing (amplitude of the signal variation) at the load, only 1.5 to 2 V is left for the actual signal processing. This also sets a lower limit on practical implementation.